In numerous applications, and in particular for wheel shafts, outlet shafts from gearboxes, or crank shafts of internal combustion engines, a rotary shaft is fitted with a device for encoding angular displacement in order to measure its instantaneous speed of rotation and/or its angular position and/or its direction of rotation.
It is advantageous to make the encoder element using a magnetizable polymer, i.e. a polymer of the elastomer type, or of any other material capable of performing a binder function, and containing a high proportion of magnetic particles, such as ferrite particles, for example. Such magnetizable polymers can be configured in a variety of shapes by molding, and they can be secured to a support by being bonded directly thereto. However, such polymers are relatively friable and their lifetime can be reduced in certain severe applications where the angle encoder is particularly exposed to abrasive particles, such as dust or sand, for example.
Proposals have been made to protect the encoder element by a metal covering part made of non-magnetic metal, which part is mounted on the encoder so as to cover at least the encoding zone, as described in US patent application No. US-A-2002/040418. Nevertheless, the additional covering part needs to be secured firmly to the encoder and increases the complexity and the cost of manufacture. Furthermore, said part needs to present thickness that is relatively large in order to have sufficient strength, thereby increasing the gap between the encoder element and the magnetic sensor, thus degrading the magnetic field emitted towards the sensor, and increasing the overall size of the encoder, given that the magnetizable polymer needs to be relatively bulky in order to generate a magnetic field that is detectable. The presence of a covering part may require the thickness of the magnetizable polymer to be increased as measured perpendicularly to the encoding zone in order to compensate for the disturbance to the magnetic field.